No woman is ever doing something wrong by not sleeping with you.
Oh no she's been friends with you for forever but you've gotten nowhere? If you're in any way offended by this, YOU'RE AN ASSHOLE.
Being your friend doesn't mean she is doing something wrong by not utilizing her body to assuage your sexual frustrations. God forbid she actually be your friend.
And no, it's not true that girls only fall for assholes and that's why it sucks being a "nice guy". If assholes got girls, YOU'D BE GETTING LAID RIGHT NOW.
If you've never made a move on that friend-girl you're in love with, this is no one's fault but yours.
If you've made a move and she says no, accept this. If you can still be her friend (for real, JUST a friend), then that's great. If you cant and you need time, that's okay too. Go off and deal with that. Do NOT wait around and keep up a constant mild pressure on her to wear her down. Be aware of this if you're doing it. Then stop it immediately.
If you were never her friend to begin with but really are just hanging around in the hopes she'll sleep with you? You're a creep.
And a note based on a recent experience:
If you've just met a girl, refrain from making sex jokes or talking about sex on the first encounter. You will look like a creeper. It's like meeting someone and telling a racist or dead baby joke right away. That person will make assumptions on your character based on what you say.
And getting mad at someone else because they've been made uncomfortable by you is pathetic.